This is simply underwater weighing and
is one of the more accurate tests for determining your body composition.

First, your dry weight is measured. First
they measure your dry weight. Next, you are lowered into a water tank
until you are completely submerged. At this point you blow out any further
air that you can. Then your underwater weight is measured.

The margin of error is around 2-3 percent
but the accuracy depends on how much air you can expel from your lungs.
You must blow it all out or it won't be as accurate as it could be. This
can be disturbing for anyone who is uncomfortable in water.